Overcharge protection systems for rechargeable batteries

ABSTRACT

Disclosed is an electrochemical device having a shuttle-type redox mechanism for overcharge protection in which the redox reaction is &#34;tuned&#34; with a tuning agent to adjust the potential at which the redox reaction occurs. Such device may be characterized as including the following elements: (1) a negative electrode (e.g., lithium); (2) a positive electrode containing one or more intermediate species (e.g., polysulfides) which are oxidized to one more oxidized species during overcharge; and (3) a tuning species (e.g., an organosulfur compound) which adjusts the rate at which the oxidized species are reduced and thereby adjusts the voltage at which overcharge protection is provided. The oxidized species produced during overcharge move to the negative electrode where they are reduced back to said intermediate species as in a normal redox shuttle. However, the oxidized species react more rapidly than the intermediate species at the negative electrode. Thus, the overcharge protection mechanism becomes more active as the oxidized species&#39; concentration increases--as occurs during more severe overcharge.

BACKGROUND OF THE INVENTION

The present invention relates to electrochemical cells having amechanism for protecting against damage from overcharge. Morespecifically, the invention relates to cells in which sulfur or asimilarly acting species is oxidized during overcharge at a positiveelectrode and then shuttles to a negative electrode where it is reduced.

Damage from overcharge presents a significant problem for many secondarybatteries. Normal recharging is intended to be carried out until thecell on recharge reaches a defined voltage. If for any reason thisvoltage is exceeded, overcharge may result. Overcharge can cause variousundesirable reactions such as destruction of the cell electrolyte,corrosion of current collectors, degradation of cell separators, andirreversible damage to the positive or negative electrode. Any one ofthese conditions can lead to the destruction of the cell. Further,overcharge can create unsafe conditions such as cell venting due toelectrolyte gassing.

The problem may be especially pronounced when a plurality ofelectrochemical cells are coupled together in series (e.g., to form abattery or battery pack). Typically, the individual cells will possessat least slightly varying capacities--reflected as the maximum number ofcoulombs (or ampere-hours) that the cell accept without overcharging.Those cells having lower capacities will become fully charged beforeother cells having higher capacities. If charging continues after thelower capacity cells reach full charge, the lower capacity cells willovercharge and possibly be damaged.

Various approaches to overcharge protection have been employed. In oneapproach, a protective additive is provided to the cell. Such additivesundergo "parasitic" reactions at cell potentials above the cell's fullcharge potential but below a destructively high cell potential. Thus,suitable additives are chosen based upon characteristic voltages atwhich they are oxidized and reduced. If an electrode voltage reaches theadditive's characteristic oxidation or reduction potential, the additivebegins to react and continues to react until the cell potential recedesto a safe level.

One widely-used class of protective additives is based on theorganometallic ferrocene compounds. A given ferrocene may be oxidized ata voltage of about 3 volts (versus a lithium negative electrode) forexample. Consider a ferrocene-protected lithium-iron sulfide cell havinga normal full charge cell potential of about 1.8 volts. When duringcharge of such cell, all positive electrode material has been fullyoxidized, the cell voltage increases beyond 1.8 volts towards 3 volts.When the cell voltage reaches 3 volts, the ferrocene additive begins toreact. Specifically, it is oxidized at the positive electrode. Theoxidized compound then travels to the negative electrode where it isreduced. The reduced compound then shuttles back to the positiveelectrode to again be oxidized. In this manner, the ferrocene provides ashuttle redox mechanism, thereby protecting the cell from attaining toohigh of a voltage.

As most widely-used redox shuttle additives, like ferrocenes, are cyclicorganic compounds, they eventually degrade under the harsh cellenvironment of a rechargeable alkali-metal cell. Thus, the protectionthey provide eventually decreases during the cell's life.

Further, because such redox additives invariably react at a givenpotential, they may sometimes impede normal cell charging. For example,during rapid charging, the cell voltage may slightly exceed the point atwhich the additive reacts, even though the cell has not been charged tofull capacity. When this occurs, the charging current shunts to theadditive's redox reaction and away from the desired charging reaction.

At least one inorganic overcharge protective additive has been employed.In the article "Overcharge Protection in Li-Alloy/Metal Disulfide Cells"by L. Redey, Proceedings--Electrochemical Society (Proc. Jt. Int. Symp.Molten Salts), 87-7, pages 631-636, (1987), molten-salt-electrolytelithium-alloy/metal-sulfide cells employing lithium sulfide as anovercharge protection agent are described. In these cells, the lithiumsulfide is soluble in molten electrolyte. During overcharge, it reactsat the positive electrode to produce a lithium polysulfide of relativelylow oxidation state. The polysulfide then shuttles to the negativeelectrode where is it reduced back to sulfide on the lithium alloy. Thesulfide additive used in this redox shuttle is more robust in the faceof lithium negative electrodes than ferrocenes and other common organicadditives. Unfortunately, the protective reaction described in the Redeyreference occurs at the rather low voltage of about 1.9 to 2.05 volts.This is lower than the operating voltage for many important cells in usetoday. Thus, application of the Redey protective mechanism is limited tocells having potentials below about 1.9 volts (e.g., the lithiumalloy-iron sulfide molten-salt cell described by Redey).

In view of the above difficulties, what is needed is an improvedovercharge protection mechanism employing protective species which (1)resist attack by lithium (or other highly reactive electrode materials),(2) are stable in both the reduced and oxidized states, and (3) do notimpede the cell's normal charge and discharge functioning.

SUMMARY OF THE INVENTION

Applicants have recognized that certain electrochemical cells, notablythe lithium-sulfur cell, possess an internal overcharge protectionmechanism. In accordance with this invention, that mechanism has beenharnessed and engineered for application to a wide range of cellsoperating under a wide range of conditions.

In lithium-sulfur cells, it has been discovered that on overchargepolysulfide species of intermediate oxidation state located in thepositive electrode are converted to more highly oxidized polysulfidespecies. These more highly oxidized species are transported to thenegative electrode where they are reduced back to the intermediatepolysulfide species. The intermediate polysulfide species so producedthen move back to the positive electrode where they are again oxidizedto the oxidized polysulfide species. By providing this steady flux ofintermediate polysulfide species at the positive electrode, the cellpotential is maintained at a relatively low level dictated by theoxidization reaction of the intermediate polysulfide species.

Key to this reaction mechanism is the relative kinetics of theintermediate and highly oxidized polysulfide reactions. The highlyoxidized species react much faster than the intermediate species (seeFIG. 3 which is described below). Thus, as overcharge conditions becomemore severe (more oxidizing cell potentials), and thereby produce morehighly oxidized polysulfide species, the rate of the protective reactionincreases. Under less extreme conditions (lower cell potentials), whenovercharge protection is less necessary, the protective reactionoperates more slowly.

In one aspect of the present invention, an electrochemical energyconversion device is provided in which a shuttle mechanism (such as theabove-described polysulfide mechanism) is "tuned" with a tuning agentwhich adjusts the potential at which the reaction occurs. Specifically,such device may be characterized as including the following elements:(1) a negative electrode; (2) a positive electrode containing one ormore intermediate species which are oxidized to one or more oxidizedspecies during overcharge; and (3) a tuning species which adjusts therate at which the oxidized species are reduced and thereby adjusts thevoltage at which overcharge protection is provided. The oxidized speciesproduced during overcharge move to the negative electrode where they arereduced back to the intermediate species as in a normal redox shuttle.However, the tuning species affects the rate at which the oxidizedspecies reacts at the negative electrode.

The tuning species tailors the overcharge protection voltage to a levelthat is appropriate for a given cell. For example, if the intermediatespecies' "native" reaction potential is lower than the normal fullycharged cell voltage, the tuning species should slow the protectant'sreaction rate to thereby increase the overcharge protection potential.Thus, the tuning species generally may be any material added to adjustthe potential at which the conversion between the intermediate speciesand the more oxidized species occurs.

In one embodiment, the tuning species is an organic sulfur compound ofthe general formulas RS and (R(S)_(y))_(n), wherein y is a value between1 and 6, n is a value between about 2 and 1000, and R is one or moredifferent aliphatic or aromatic organic moieties having between 1 andabout 20 carbon atoms, which may include one or more oxygen, sulfur, ornitrogen heteroatoms when R comprises one or more aromatic rings, or oneor more oxygen, sulfur, nitrogen, or fluorine atoms associated with thechain when R includes an aliphatic chain, wherein the aliphatic groupmay be linear or branched, saturated or unsaturated, and wherein eitherthe aliphatic chain or the aromatic ring may have substituted groupsthereon. Representative compounds include trithiocyanuric acid,thiophene, tetraethylthiuram disulfide ((C₂ H₅)₂ NC═SS)₂, polyethylenedisulfides (e.g., (SCH₂ CH₂ S)_(n), and C₂ H₅ S--SC₂ H₅), and mercaptans(e.g., CH₃ SH).

In a second embodiment, the tuning species is an alloying element addedto a primary metal component of the negative electrode. For example,aluminum, silicon, magnesium, or manganese may be added to a lithiummetal negative electrode.

In a third embodiment, the tuning species is a "non-reactive" surfaceactive agent selected from the group consisting of boron containingcompounds including organoborates such as trimethylborate, boroxines,such as trimethylboroxine, phosphorus containing compounds includingpolyphosphazenes and phosphates such as Li₃ PO₄, carbonates such as Li₂CO₃, nitrogen containing compounds including nitrates such as LiNO₃ andorganonitrogen compounds such as phenylhydrazine.

In a fourth embodiment, the tuning species is an electrolyte additivewhich affects the solubility of the intermediate species and/or oxidizedspecies. If the intermediate and oxidized species are made less soluble(by choosing an appropriate electrolyte), then the potential at whichovercharge protection occurs is increased. If the intermediate speciesare made more soluble (again by choosing an appropriate electrolyte),then the overcharge protection potential is lowered. Electrolytes inwhich polysulfides, for example, are rather soluble include amides suchas acetamide, dimethylacetamide, and 1-methyl-2-pyrollidinone, ketonessuch as cyclohexanone, lactones such as γ-butyrolactone andγ-valerolactone, sulfones such as sulfolane and 2,4-dimethylsulfolane,sulfoxides such as methyl sulfoxide and tetramethylene sulfoxide,carbonates such as propylene carbonate, ethylene carbonate, diethylcarbonate and dimethyl carbonate, ethers including the ethoxyethers suchas the glymes CH₃ O(CH₂ CH₂ O)_(n) CH₃ where n=1 to 5 includingpenta-glyme CH₃ O(CH₂ CH₂ O)₅ CH₃, tetra-glyme CH₃ O(CH₂ CH₂ O)₄ CH₃,tri-glyme CH₃ O(CH₂ CH₂ O)₃ CH₃, di-glyme CH₃ O(CH₂ CH₂ O)₂ CH₃, andmono-glyme CH₃ OCH₂ CH₂ OCH₃, the polyglymes CH₃ O(CH₂ CH₂ O)_(n) CH₃where n is between about 6 and 100, the polyoxyethers such aspolyethylene oxide (CH₂ CH₂ O)_(n) where n is between about 100 and200,000, and cyclic ethers such as tetrahydrofuran ("THF"), and2,5-dimethyltetrahydrofuran.

Electrolytes in which polysulfides, for example, are rather insolubleinclude alkanes such as hexane, heptane, and octane, monocyclicaromatics such as benzene, toluene and xylene, and polycyclic aromaticssuch as napthalene and perylene. To precisely tune the overchargeprotection potential, the electrolyte preferably includes in acombination of one of the poor solvents with one of the good solvents.

In a preferred embodiment, the rechargeable electrochemical energyconversion device includes sulfur or an organosulfide compound as thepositive electrode and an alkali or alkaline earth metal as the negativeelectrode. In the case of a sulfur cell, the intermediate and oxidizedspecies may both be polysulfides of the formula M_(y) S_(x), where y=1or2, x is greater than or equal to 2, and where the value of x is greaterin the oxidized species than in the intermediate species. Preferably,the value of x is greater than or equal to 6 for the oxidized species.In many such sulfur cells, a sulfide film of formula M_(y) S forms onthe negative electrode (e.g., a lithium sulfide film on a negativeelectrode of lithium or a lithium alloy).

While tuned overcharge protection systems of this invention may beapplied to many different cells, they are preferably applied to cells inwhich the negative electrode is an alkali metal including lithium andits alloys such as lithium aluminum alloys, lithium silicon alloys, andlithium tin alloys; sodium and its alloys such as sodium lead alloys;alkaline earth electrodes such as magnesium and their alloys; transitionmetal electrodes such as aluminum, zinc, and lead and their alloys;intercalation anodes such as Li_(x) C₆ ; glass matrix electrodes such asLi/Sn₂ O₃ and Li/SiO₂. Such electrodes may be applied in various cellsincluding lithium/organosulfur cells such as Li/(SCH₂ CH₂ S)_(n) ;lithium/(inorganic sulfur) cells such as Li/Li₂ S_(x) ; lithium/(metaloxide) cells such as Li/Li_(x) Mn₂ O₄ and Li/V₆ O₁₃, lithium/(metalsulfides) cells such as Li/TiS₂ and Li/MoS₂ ; and carbon anode cellssuch as Li_(x) C₆ /Li_(x) CoO₂.

In another aspect, the present invention provides rechargeableelectrochemical energy conversion devices in which a sulfur species isprovided as an additive to a cell such that the above-describedpolysulfide shuttle mechanism protects the cells against overcharge.Specifically, such electrochemical energy conversion devices may becharacterized as including the following elements: (1) a positiveelectrode other than a sulfur electrode; (2) a negative electrode; (3) asulfur-based additive; and (4) an electrolyte in which polysulfidespecies are substantially soluble and sulfide species are substantiallyinsoluble. The sulfur-based additive includes at least one of elementalsulfur, a sulfide species, and one or more polysulfide species. Duringovercharge in such cell, a polysulfide species is oxidized to anoxidized polysulfide species which is subsequently reduced by a reactionwith the substantially insoluble sulfide species.

Preferably, the polysulfide species has the formula M_(y) S_(x) asdefined above. Given that the untuned polysulfide shuttle reactionbecomes significant at cell potentials around 2.2 to 2.4 volts,electrochemical devices to which the sulfur-based additive is providedpreferably have fully charged voltages of at most about 2.5 volts. Ofcourse, a tuning species (as described above) may be employed to adjustthe potential at which the oxidized polysulfide species is reduced andthereby adjusts the voltage at which overchargeable protection isprovided.

Some preferred devices to which the sulfur-based additive may beprovided are polymeric cells (having a polyethylene oxide electrolytefor example). Further, to ensure that the sulfide species remainsinsoluble in the electrolyte, the device is preferably operated at atemperature of at most about 200° C. (or, in lithium cells, about 180°C.--the melting point of lithium metal).

For this aspect of the invention, suitable negative electrodes includean alkali metal including lithium and its alloys such as lithiumaluminum alloys, lithium silicon alloys, and lithium tin alloys; sodiumand its alloys such as sodium lead alloys; alkaline earth electrodessuch as magnesium and their alloys; transition metal electrodes such asaluminum, zinc, and lead and their alloys; intercalation anodes such asLi_(x) C₆ ; glass matrix electrodes such as Li/Sn₂ O₃ and Li/SiO₂.Suitable positive electrodes include metal oxides such as MoO₂, MoO₃,WO₂, and V₆ O₁₃ ; metal sulfides such as NiPS₃, TiS₂, and VS₂, andorganosulfur electrodes such as (SCH₂ CH₂ S)_(n), ((C₂ H₅)₂ NC═SS)₂, andC₂ H₅ S--SC₂ H₅. Thus, cells which may benefit from the sulfur-basedadditive of this invention include lithium/metal oxide cells such asLi/MoO₂ and Li/V₆ O₁₃ ; lithium/metal sulfides such as Li/TiS₂ cells;lithium/organosulfur cells such as Li/(SCH₂ CH₂ S)_(n) ; carbon anodecells such as Li_(x) C₆ /TiS₂ and glass matrix anode cells such as(Li/Sn₂ O₃)/TiS₂.

Another aspect of the present invention pertains to methods of using theabove-described cells to protect against damage from overcharge.

These and other features and advantages of the invention will bedescribed in more detail below with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an arrangement of cell components for alithium cell which may be employed in a preferred embodiment of thepresent invention.

FIG. 2 is a mechanistic representation of an internal polysulfideovercharge protection mechanism of a lithium-sulfur cell.

FIG. 3 is a graph showing how reaction rate increases with oxidationlevel of polysulfides (or other appropriate overcharge protectant) inaccordance with the present invention.

FIG. 4 is a mechanistic representation of an internal overchargemechanism believed to exist in lithium-organodisulfide cells.

FIG. 5 is an illustration of the potential at which various cellmechanisms of relevance to the present inventions occur.

FIG. 6 is a graph showing how reaction rate abruptly increases withoxidation level in a preferred battery which exhibits a low selfdischarge rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

General Points

The present invention provides an overcharge protection system andmethod for recharging electrochemical cells. In the followingdescription, numerous specific details of the cells and mechanisms areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, that the present invention maybe practiced without limitation to some of the specific detailspresented herein.

Some suitable materials for use in fabricating certain sulfur-basedcells of this invention, as well as fabrication methods employedtherewith, are described in U.S. Pat. Nos. 5,523,179 (issued Jun. 4,1996) and U.S. Pat. No. 5,582,623 (issued Dec. 10, 1996) as well as U.S.patent application Ser. No. 08/686,609 filed Jul. 26, 1996 (attorneydocket no. PLUSP003). Each of these patent documents names May-Ying Chuas inventor, is assigned to the assignee of the present invention, andis incorporated herein by reference for all purposes.

Referring now to FIG. 1, a cell laminate 10 in accordance with apreferred embodiment of the present invention is shown. Laminate 10includes a negative current collector 12 which is preferably formed of ametallized polyester strip. The metallized portion of this elementserves to conduct electrons between a cell terminal (not shown) and anegative electrode layer 14 (such as lithium) to which current collector12 is affixed. The bottom portion of negative electrode 14 is aseparator/electrolyte layer 16. Preferably, the separator is a polymersuch as polyethylene oxide. Affixed to the bottom of separator layer 16is a positive electrode layer 18. As layer 16 is chosen to be anelectronic insulator and ionic conductor, positive electrode 18 isionically coupled to but electronically insulated from negativeelectrode 14. Finally, the bottom of positive electrode layer 18 isaffixed to a current collector layer 20. Layer 20 provides an electronicconnection between a positive cell terminal (not shown) and positiveelectrode 18. Preferably, layer 20 is a metallized polyester currentcollector (like layer 12).

The choice of terms "top" and "bottom" to describe cell laminate 10 wasmade for ease of description. These terms in no way limit theorientation of the cell or its components.

Cell laminate 10 may be conveniently fabricated by current batch orcontinuous thin layer techniques such as those described in U.S. Pat.No. 5,582,623, previously incorporated by reference. In a continuousfabrication technique, the a slurry of positive electrode material maybe continuously applied by a suitable coating apparatus, such as adoctor blade, to a sheet of current collector. The resulting positiveelectrode/current collector may be bonded to the other layers of thecell by continuously providing the individual layer to a set of rollersoperated at a sufficiently high temperature to bond the individuallayers one to another. Using such techniques, which are well known inthe art, laminate 10 may be produced with a total thickness on the orderof 100 micrometers or thinner.

Cell laminate 10 is but one of many cell designs to which the presentinvention may be employed. Other appropriate cell designs includespirally wound ("jelly roll") designs, prismatic designs, coin celldesigns, etc. Methods of preparing each of these cell types are wellknown in the art. Further, any of the electrodes and electrolytes may besolids, gels, or liquids.

Referring now to FIG. 2, a lithium-sulfur cell 40 is shown with relevantovercharge protection mechanisms depicted. Cell 40 includes a lithiummetal negative electrode 30, a polyethylene oxide separator/electrolyte34, and a sulfur positive electrode 36. Lithium electrode 30 includes alithium sulfide (Li₂ S) passivation layer 32 formed adjacent toseparator 34. During normal charge, the electrons are extracted frompositive electrode 30 and transported over an electrical connection 38to negative electrode 30. The removal of electrons at positive electrode36 oxidizes the species present in the electrode. In this reaction,lithium ions are liberated from lithium sulfide and/or lithiumpolysulfide species present in the positive electrode. The speciesremaining in the positive electrode will have the general formula Li₂S_(x), where x has a value of 2 or greater. Over time the chargereaction produces polysulfide species having longer and longer sulfurchains. It is known for example that in a normal charge reaction, thevalue of x in some polysulfides may be 12 or greater.

At the negative electrode, lithium ions present in the electrolyte 34and passivation layer 32 are reduced to lithium metal as electrons areprovided to negative electrode 30 through electrical conduit 38.

At a fully charged cell potential (typically in the neighborhood of 2.2to 2.4 volts), charging normally ceases. If charging could continue, thecell would overcharge with the potential continuing to increase to alevel where deleterious side reactions occur. However, the internalovercharge protection mechanism of the lithium-sulfur cell normallyprevents such deleterious reactions. Specifically, as charging currentis continually introduced into positive electrode 36, more highlyoxidized lithium polysulfide species are produced. In FIG. 2, thesespecies are represented by the formula Li₂ S_(x+)δ.

As these highly oxidized polysulfide species are generated, theirconcentration increases in the vicinity of the positive electrode 36. Asa result, a concentration gradient is established and the soluble highlyoxidized species are driven away from positive electrode 36 towardpassivation layer 32 by diffusion. Other transport mechanisms mayfacilitate movement of these polysulfides. When the highly oxidizedpolysulfide species reach passivation layer 32, they react withinsoluble lithium sulfide (Li₂ S) to produce intermediate oxidizedspecies of formula Li₂ S_(x). As these intermediate polysulfide speciesbuildup in the vicinity of passivation layer 32, they move back topositive sulfur electrode 36. There, the intermediate species may bereoxidized to oxidized polysulfide species (Li₂ S_(x+)δ) if overchargeconditions persist. This polysulfide redox shuttle mechanism appears tobe intrinsic to lithium-sulfide cells in which polysulfides are solubleto some extent in the electrolyte.

The more highly oxidized polysulfide species react faster than the lessoxidized species (i.e., lithium sulfide and the intermediate oxidizedlithium polysulfides). FIG. 3 presents a curve showing how reaction rateof a polysulfide varies as a function of the polysulfide anion'sstoichiometric coefficient, x+δ. As the value of x+δ increases beyond acertain point, the reaction rate quickly climbs to a very high level.Thus, as overcharge conditions become more severe and the value of x+δincreases, the overcharge protection reaction speeds up to counter theeffects of overcharge.

Note that the less oxidized species react quite slowly, if at all, withthe negative electrode. If this were not the case, the cell mightcontinuously self discharge after charging. The soluble charged speciesfrom the positive electrode would simply move to the negative electrodewhere they would be reduced and therefore unavailable for participationin the electrochemical reactions necessary to produce electrical energy.

Note also that the highly oxidized polysulfide species react readilywith lithium metal as well as less oxidized polysulfides. As a result,the highly oxidized polysulfides may scavenge lithium metal dendritesthat may form and break off during normal cycling. As understood in thefield, such metallic dendrites are sometimes responsible for shorteningthe life of rechargeable lithium metal cells. Because the polysulfidespecies chemically react with lithium metal and thereby reintroduces itto the cell's productive electrochemical cycle, the cells of thisinvention generally have longer cycle lives than comparable lithiummetal cells.

Regarding the species involved in the protective redox shuttle, it isbelieved that the more oxidized overcharge products from the positiveelectrode are lithium polysulfides in which x is greater than 3 (or(x+δ)>3 in the representation employed in FIGS. 2 and 3) and more likelyin which 6<x <20. The reactivity of the polysulfides appears to increasesignificantly when x>6.

As the above mechanism is based upon a shuttling between intermediateand oxidized polysulfide species, it operates at a higher potential thanmechanisms shuttling between lithium sulfide and lithium polysulfides(the shuttle described in the Redey article). Note that in the moltensalt electrolyte systems employed by Redey, lithium sulfide, as well aslithium polysulfides, are extremely soluble. In the systems of thisinvention, lithium sulfide (and possibly some of the less oxidizedpolysulfides) will generally be insoluble and remain as part ofpassivation layer 32 (at least during the critical overcharge stage).

Further, the above-described polysulfide mechanism should be contrastedwith conventional overcharge protection schemes in which a parasiticadditive is provided to the cell. Such additives are chosen based uponcharacteristic voltages at which they are oxidized and reduced. Theprotection mechanisms of the present invention are, in contrast, basedupon a composition rather than voltage. Only after all sulfur speciesare sufficiently oxidized (e.g., with x being approximately 6 orgreater), will the overcharge mechanism of this invention be activated.Thus, the problem of shunting charging current to a parasitic redoxreaction during rapid charging will not arise in the compositiondependent mechanism of the present invention. As long as there isadditional active-sulfur in a limited oxidation state, the chargingreaction will proceed--regardless of whether the cell voltage slightlyexceeds the normal fully charged cell voltage.

Generally, any cell species (polysulfide or otherwise) having a seriesof oxidization states, in which the lower oxidation states react moreslowly than the higher more oxidized species, will provide the desiredinherent overcharge protection mechanism. Thus, suitable cells for usewith this invention include lithium/organosulfur cells such as Li/(SCH₂CH₂ S)_(n) ; lithium/(inorganic sulfur) cells such as Li/Li₂ S_(x) ;lithium/(metal oxide) cells such as Li/Li_(x) Mn₂ O₄ and Li/V₆ O₁₃,lithium/(metal sulfides) cells such as Li/TiS₂ and Li/MoS₂ ; and carbonanode cells such as Li_(x) C₆ /Li_(x) CoO₂.

In general, oxidized overcharge products produced in the positiveelectrode near or at the end of the charging cycle travel to thenegative electrode and there react at the electrode surface. In manycells, these overcharge products are reactive with the a passivationlayer (on the negative electrode surface) which consists, in part orwhole, of less oxidized species. A reaction between the more oxidizedovercharge products and the passivation layer compounds reduces theovercharge products. Reduced species may then travel to the positiveelectrode where they may be reoxidized before returning to the negativeelectrode. Thus, it appears that a redox shuttle mechanism thatgradually removes or reduces the thickness of the negative electrodepassivating layer protects the battery from overcharge.

Tuning the Potential at which Overcharge Protection is Triggered

As noted, the intrinsic overcharge protection mechanism afforded by theabove-described polysulfide shuttle is provided at about 2.2 to 2.4volts. This voltage is dictated by the shape of the curve shown in FIG.3. In a simple lithium-sulfur cell, the intrinsic reaction rate curvemay take the form of curve 60. This reaction curve will have aparticular voltage associated therewith by virtue of the speed at whichoxidized polysulfide species react. If the curve is shifted to the rightas shown by curve 62, no significant reaction would occur until evenmore highly oxidized polysulfide species were produced. As the morehighly oxidized polysulfide species are generated only at higherovercharge voltages, the potential associated with curve 62 would behigher than the potential associated with curve 60. If a cell additivecorresponding to curve 60 provides an overcharge protection mechanismtriggered at 2.2 to 2.4 volts, the overcharge mechanism associated withcurve 62 might be triggered at a potential of 2.6 to 2.8 volts.

Similarly, if curve 60 is shifted to the left as shown with curve 64,less oxidized polysulfide species will react more rapidly. As the lessoxidized species are produced at lower voltages (than the speciesassociated with curve 60), a cell exhibiting curve 64 would have anovercharge protection mechanism triggered at a relatively low voltage(e.g., 1.8 to 2.0 volts).

In a preferred embodiment, a "tuning species" is added to anelectrochemical energy conversion device of this invention in order toshift a cell's intrinsic reaction rate curve (e.g., curve 60 of FIG. 3)to the left or right. This controls the potential at which theovercharge protection mechanism is triggered. While many differenttuning species may be appropriate for adjusting the voltage in a desireddirection, those of skill in the art will understand that a suitabletuning species for a particular application may be identified withroutine screening techniques.

Further, careful choice of solvent can tune the potential at whichovercharge protection begins. In general, solvents which increase thesolubility of Li₂ S_(x) species (with low x values) lower the potentialat which overcharge protection begins. In contrast, solvents whichdecrease the solubility of such species, increase the potential at whichovercharge protection begins. This can be understood by realizing thatuntil a species is soluble in the electrolyte, it cannot shuttle betweenelectrodes and provide overcharge protection. Thus, if a polysulfide(say Li₂ S₃) produced at the negative electrode is insoluble, it cannotmove to the positive electrode and provide overcharge protection. Onlywhen the soluble more oxidized species are produced at the negativeelectrode (because even more oxidized species are produced at thepositive electrode) can the overcharge protection begin. Thus, theovercharge protection potential is relatively high.

To further illustrate this concept, consider the following list ofsulfide/polysulfide reactions at the negative electrode:

Li₂ S+Li₂ S₈ =Li₂ S₅ +Li₂ S₄ (soluble)

Li₂ S+Li₂ S₇ =2Li₂ S₄ (soluble)

Li₂ S+Li₂ S₆ =Li₂ S₄ +Li₂ S₃ (insoluble)

Li₂ S+Li₂ S₅ =2Li₂ S₃ (insoluble)

This list is provided for an arbitrary solvent that solubilizes Li₂ S₄,but does not solubilize Li₂ S₃. As potential rises at the positiveelectrode, some Li₂ S₅ is formed and dissolves in the electrolyte. Itthen moves to the negative electrode where it reacts with the Li₂ S filmon the lithium electrode to form insoluble Li₂ S₃ which is unavailableto shuttle to the positive electrode and oxidize. As potential risesfurther, some Li₂ S₆ forms at the positive electrode and moves over tothe negative electrode where it reacts with the Li₂ S film on thelithium electrode to form insoluble Li₂ S₃ and soluble Li₂ S₄. Thus,some Li₂ S₄ can shuttle back to the positive electrode for oxidation.The Li₂ S₃ is of course unavailable for reaction.

Finally, as the potential rises to the level where Li₂ S₇ and moreoxidized species form, these species move from the positive electrode tothe negative electrode where they form less oxidized but solublepolysulfides as indicated by the top two reaction equations above. Thus,the cell potential should stabilize.

If the electrolyte is modified such that Li₂ S₄ is insoluble, then thestable overcharge potential will be higher. If on the other hand, theelectrolyte solubilizes Li₂ S₃, the stable overcharge potential will belower.

It should be understood that the above reaction list is not exhaustive.As lithium compounds other Li₂ S will be present on the negativeelectrode surface, these other compounds will participate in the localreactions. Also, many non-stoichiometric reactions should be considered.

This invention provides cells having electrolytes chosen to tune thepotential at which overcharge protection begins. Effective solvents forLi₂ S_(x) tend to have large donor numbers ("DN") as measured by theGutmann Donor Number; this measures the ionizing strength of a solvent.In addition, good solvents for Li₂ S_(x) need to have strongdissociating power as indicated by the dielectric constant (ε). Forexample, diethylacetamide has both a high donor number (DN=27.8) and alarge dielectric constant (ε=37.8) and, as expected, is an effectivesolvent for Li₂ S_(x). Tetrahydrofuran has a relatively low dielectricconstant (ε=7.58), but a large donor number (DN=20.0), and is a goodsolvent for Li₂ S_(x). Unfortunately, donor numbers and dielectricconstants may not be available for many potential solvents andnon-solvents. Therefore, in many cases solubility must be experimentallydetermined.

Appropriate electrolytes can be identified by measuring the solubilityof various polysulfides in chosen solvents. Fielder and Singer("SOLUBILITY, STABILITY, AND ELECTROCHEMICAL STUDIES OF SULFUR-SULFIDESOLUTIONS IN ORGANIC SOLVENTS", NASA, Scientific and TechnicalInformation Office (1978)) presents solubilities of sodium polysulfidesin various solvents. This reference is incorporated herein by referencefor all purposes.

Electrolytes in which polysulfides, for example, are rather solubleinclude amides such as acetamide, dimethylacetamide, and1-methyl-2-pyrollidinone, ketones such as cyclohexanone, lactones suchas γ-butyrolactone and γ-valerolactone, sulfones such as sulfolane and2,4-dimethylsulfolane, sulfoxides such as methyl sulfoxide andtetramethylene sulfoxide, carbonates such as propylene carbonate,ethylene carbonate, diethyl carbonate and dimethyl carbonate, ethersincluding the ethoxyethers such as the glymes CH₃ O(CH₂ CH₂ O)_(n) CH₃where n=1 to 5 including penta-glyme CH₃ O(CH₂ CH₂ O)₅ CH₃, tetra-glymeCH₃ O(CH₂ CH₂ O)₄ CH₃, tri-glyme CH₃ O(CH₂ CH₂ O)₃ CH₃, di-glyme CH₃O(CH₂ CH₂ O)₂ CH₃, and mono-glyme CH₃ OCH₂ CH₂ OCH₃, the polyglymes CH₃O(CH₂ CH₂ O)_(n) CH₃ where n is between about 6 and 100, thepolyoxyethers such as polyethylene oxide (CH₂ CH₂ O)_(n) where n isbetween about 100 and 200,000, and cyclic ethers such as tetrahydrofuran("THF"), and 2,5-dimethyltetrahydrofuran.

Electrolytes in which polysulfides, for example, are rather insolubleinclude alkanes such as hexane, heptane, and octane, aromatics such astoluene and xylene, and polycyclic aromatics such as napthalene andperylene. Preferably the electrolyte includes in a combination of one ofthe poor solvents with one of the good solvents.

To tune accurately solubility, the electrolyte preferably includes atleast one good solvent and at least one poor solvent. Thus, in apolysulfide cell, the electrolyte preferably includes a solvent from theabove list of good solvents and a solvent from the list of poorsolvents. This is contrary to most cell design criteria which suggestthe use of only good solvents. Note that the chosen electrolyte shouldnot solubilize lithium sulfide.

One specific example of a tuning species is an alloying element added toa metal negative electrode. For example, magnesium or aluminum may beadded a lithium negative electrode in a concentration of about 10 to 20percent by weight. Such alloying elements will generally decrease thecell potential and therefore increase (relative the cell potential) theovercharge protection potential. They may also speed up or slow down thespeed at which the polysulfides react at the negative electrode.

Another example of a tuning agent is a surface active agent whichparticipates in the reduction reaction. For example, any of a number oforgano-sulfur compounds may be used to tune the reaction rate. Thefollowing organo-sulfur compounds can be expected to affect the reactionrate (thereby raising or lowering the overcharge protection potential):compounds of the general formulas RS and (R(S)_(y))_(n), wherein y is avalue between 1 and 6, n is a value between about 2 and 1000, and R isone or more different aliphatic or aromatic organic moieties havingbetween 1 and about 20 carbon atoms, which may include one or moreoxygen, sulfur, or nitrogen heteroatoms when R comprises one or morearomatic rings, or one or more oxygen, sulfur, nitrogen, or fluorineatoms associated with the chain when R includes an aliphatic chain,wherein the aliphatic group may be linear or branched, saturated orunsaturated, and wherein either the aliphatic chain or the aromatic ringmay have substituted groups thereon. Representative compounds includetrithiocyanuric acid, thiophene, tetraethylthiuram disulfide ((C₂ H₅)₂NC═SS)₂, polyethylene disulfides (e.g., (SCH₂ CH₂ S)_(n), and C₂ H₅S--SC₂ H₅), and mercaptans (e.g., CH₃ SH). It should be understood thatmany of these additives actually shuttle or cycle between the positiveand negative electrodes in manner analogous to the polysulfide species.

Further, these species may have a profound effect on the surfacechemistry of the negative electrode. Consider the following reaction atthe negative electrode:

Li₂ S+RSSR=LiSSR (soluble)+LiSR (insoluble).

If an insoluble LiSR film is formed on the electrode surface, it willaffect the reactivity of the polysulfides with the electrode film andthereby shift the overcharge potential.

Another class of tuning agent is the surface active agents that do notparticipate in the shuttle reaction. Examples of such surface activeagents which increase or decrease a reaction rate include surface activeagent selected from the group consisting of boron containing compoundsincluding organoborates such as trimethylborate, boroxines, such astrimethylboroxine, phosphorus containing compounds includingpolyphosphazenes and phosphates such as Li₃ PO₄, carbonates such as Li₂CO₃, nitrogen containing compounds including nitrates such as LiNO₃ andorganonitrogen compounds such as phenylhydrazine.

As noted, the above-described tuned overcharged protection mechanism canbe employed in systems other than the polysulfide system. In general,any cell having a species produced on overcharge that has multipleoxidation states in which the speed of reaction increases withincreasing oxidation state can be employed with the present invention.One class of cells meeting this requirement is thelithium-organodisulfide cell. Turning now to FIG. 4, an organodisulfidecell 80 includes a lithium negative electrode 82, an organodisulfidepositive electrode 84, and a polymeric separator 86 disposed betweennegative electrode 82 and positive electrode 84. During charge,electrons are removed from positive electrode 84 and supplied toelectrode 82 over an electrical conduit 88. As part of the normal chargeprocess, lithium ions are liberated from lithium organodisulfidecompounds in positive electrode 84. The resulting lithium ions aretransported to negative electrode 82 where they are reduced to lithiummetal. The organodisulfide anions at the positive electrode form chainsof organodisulfide polymers. Upon reaching overcharge, organodisulfidespecies of a certain relatively high oxidation state begin reacting withnegative electrode 82 where they are reduced to a less oxidized species.Generally, the reduced species will simply be a shorter chainorganodisulfide polymers.

Most likely, the lithium negative electrode 82 includes a passivationlayer 90 similar to layer 32 shown in the lithium-sulfur cell of FIG. 2.Upon being reduced at surface film 90, the organodisulfide anion movesback to positive electrode 84 where it can be reoxidized if overchargeconditions persist. Thus, the organodisulfide shuttle mechanism actsmuch like the above-described polydisulfide shuttle mechanism. It isbelieved that the reaction rate of the organodisulfide anions increaseswith oxidation state, just as in the case of polysulfide anions.

While not wishing to bound by theory, it is believed that the largehighly oxidized polyorganodisulfide anions produced during overchargemay form circular or cyclic structures. This would allow them tomaintain a high degree of mobility even though their chain length hassubstantially increased during charging. Regardless of whether or notthis is the actual physical mechanism, the highly oxidized speciesproduced during overcharge should be able to travel across the cellseparator to the negative electrode.

Other systems in addition to the lithium-sulfur andlithium-organodisulfide cells having the necessary reaction rate profilefor oxidized species include lithium/(metal oxide) cells such asLi/Li_(x) Mn₂ O₄ and Li/V₆ O₁₃, lithium/(metal sulfides) cells such asLi/TiS₂ and Li/MoS₂ ; and carbon anode cells such as Li_(x) C₆ /Li_(x)CoO₂.

Preferably, the overcharge protection of this invention limits the cellvoltage during overcharge to a safe level that does not substantiallyexceed the normal fully charged cell voltage. Thus, the overchargevoltage should remain below the level at which damage is done to cellcomponents. For example, the overcharge voltage should not cause (i) theelectrolyte to electrolyze, (ii) the current collectors to corroderapidly, (iii) the cell separator to degrade rapidly, and (iv) thepositive electrode to be irreversibly damaged. Preferably, theovercharge cell voltage will not exceed the normal fully charged cellvoltage by more than about 4 volts, more preferably by not more thanabout 2 volts, and most preferably by not more than about 1 volt.

It should be understood that the value of the "fully charged cellvoltage" is not necessarily constant between any two similarlyconstructed cells or is even constant for a given cell over that cell'slife. Obviously, there will be some chemical and/or structuralvariations from cell to cell that will cause the fully charged cellvoltage to vary. In addition, metal-sulfur cells sometimes exhibitgradual (or abrupt) changes in cell voltage over normal cycling. In allcases, the overcharge protection afforded by the present invention canbe characterized as a limitation in the deviation from the value of thefully charged cell voltage.

Sulfur Additives for Providing Overcharge Protection

Cells that might not otherwise have robust overcharge protectionmechanisms can employ sulfur-based additives in accordance with thisinvention to provide suitably robust overcharge protection. Thepolysulfide overcharge shuttle described above may be employed toprovide overcharge protection triggered at a potential centered around2.2 to 2.4 volts (versus lithium metal). This voltage can be tunedupward or downward by suitable tuning additives as described above.These additives are simply provided along with the sulfur-based materialas an overcharge protection additive (or as part of the electrodes asappropriate). If a given cell requiring overcharge protection has anormal fully charged cell potential slightly below that of a tunedpolysulfide shuttle, overcharge protection may be afforded by addingsulfur, a sulfide, a polysulfide (i.e., the sulfur-based additives), andpossibly one or more suitable tuning agents to the cell.

Generally, the parameters described above for the tuned overchargeprotection embodiment (especially as applied to polysulfide systems)apply equally to the sulfur-based additive embodiment described here.Thus, within the cell, the sulfur-based species will adopt the formulaM_(y) S_(x) as defined above, with the value of x varying as the speciesundergoes redox reactions in accordance with the overcharge protectionmechanisms. Note, however, that the sulfur must be supplied as anadditive and does automatically form from the electrode materials duringcharge/overcharge. Further, the sulfur-based additives may be providedwith or without a tuning agent.

FIG. 5 depicts potential ranges of interest for lithium-sulfur andlithium-iron disulfide cells. The lithium-sulfur cell has a fullycharged cell potential near 2.1 volts. To the extent sulfur orpolysulfides participate in the overcharge protection mechanism, thesecompounds afford protection at up to about 2.4 volts. Thus, polysulfideis an adequate overcharge protectant for lithium-sulfur cells. Ofcourse, sulfur is inherently present in such cells and so need not beadded separately. Lithium/(iron disulfide) cells have a cell potentialof about 1.8 volts (varying from about 1.8 volts on full charge to about1.5 volts on discharge) as shown in FIG. 5. Thus, the polysulfideshuttle, centered around 2.2 to 2.4 volts, will provide overchargeprotection without interfering in the normal functioning of thelithium/iron disulfide cell. It may be desirable to lower the potentialof the polysulfide shuttle to a level closer that of iron-disulfide.Thus, one embodiment of the present invention includes a lithium/irondisulfide cell having as additives a small amount (e.g., several mAhrs)of sulfur/polysulfide and a tuning agent for speeding up the polysulfidereduction reaction to thereby lower the potential at which overchargemechanism is triggered.

The Li₂ S_(x) additive will react at the lithium electrode surface toproduce the appropriate reduced Li₂ S_(x) film (insoluble). On charge,the cell will charge to full capacity, then climb to the voltage of theovercharge shuttle (e.g., about 2.2 volts), and continuously overchargeat a flat potential until cessation of charging.

FIG. 5 also illustrates that certain alloys of lithium such as lithiumaluminum have a potential slightly higher than of un-alloyed lithiummetal. Such negative electrodes, when coupled with a positive electrodeof suitable potential, may also be protected with the polysulfideadditive of this invention.

Cells which can profit from sulfur-based additives in accordance withthis invention may have such negative electrodes as an alkali metalincluding lithium and its alloys such as lithium aluminum alloys,lithium silicon alloys, and lithium tin alloys; sodium and its alloyssuch as sodium lead alloys; alkaline earth electrodes such as magnesiumand their alloys; transition metal electrodes such as aluminum, zinc,and lead and their alloys; intercalation anodes such as Li_(x) C₆ ;glass matrix electrodes such as Li/Sn₂ O₃ and Li/SiO₂. The glass matrixelectrodes are described in various references such as Tahara et al.,European Patent Application No. 93111938.2 (1993), Idota et al. CanadianPatent Application, 21134053 (1994), and I. Courtney et al. MeetingAbstacts of the Electrochemical Society, Fall Meeting, San Antonio,Tex., Oct. 6-11, 1996 Vol. 96-2, Abstract #66, page 88, each of which isincorporated herein by reference for all purposes. Positive electrodessuitable for use with these negative electrodes include metal oxidessuch as MoO₂, MoO₃, WO₂, and V₆ O₁₃ ; metal sulfides such as NiPS₃,TiS₂, and VS₂, and organosulfur electrodes such as (SCH₂ CH₂ S)_(n),((C₂ H₅)₂ NC═SS)₂, and C₂ H₅ S--SC₂ H₅. Thus, cells which may benefitfrom the sulfur-based additive of this invention include lithium/metaloxide cells such as Li/MoO₂ and Li/V₆ O₁₃ ; lithium/metal sulfides suchas Li/TiS₂ cells; lithium/organosulfur cells such as Li/(SCH₂ CH₂ S)_(n); carbon anode cells such as Li_(x) C₆ /TiS₂ and glass matrix anodecells such as (Li/Sn₂ O₃)/TiS₂. The electrodes are preferably solids orgels.

All cell components may have various forms. For example, the electrolytemay be a polymer (e.g., polyethylene oxide), a gel, or a liquid.Examples of suitable electrolytes include but are not limited to organiccarbonates such as propylene carbonate, ethylene carbonate, and dimethylcarbonate; ethers such as tetrahydrofuran, dimethyl ether, polyethyleneoxides of high and low molecular weights (i.e., glyme, di-glyme,tri-glyme, and polyethylene oxide, and other polymeric ethers such asoxymethylene linked polyoxyethylene and methoxyethoxy polyphosphazene;sulfoxides such as dimethyl sulfoxide; sulfones such as dimethylsulfone;acetamides such as dimethylacetamide; aromatics such as toluene andxylene; solvent mixtures, and solvents and/or solvent mixtures wheregelling agents are included to induce gelation of the solvent system.

It should be noted that the sulfur/polysulfide additives of the presentinvention provide a higher voltage overcharge protection mechanism thanthe sulfide/polysulfide system described in the Redey publicationdescribed above. In the molten salt system employed by Redey, lithiumsulfide is soluble and automatically reacts at the positive electrode toproduce a polysulfide of relatively low oxidation state. This low leveloxidized species then reacts at the negative electrode to be reducedback to sulfide. This shuttle occurs at an inherently lower voltage thanthe intermediate/highly oxidized polysulfide shuttle of the presentinvention. The present invention has the further advantage of beingapplicable to commercially important cells operating at temperaturesbelow 200° C. (e.g., room temperature).

A sulfur-based additive of this invention may be added to a cell byvarious techniques. For example, a lithium electrode is coated with athin film of sulfur which converts to Li₂ S. Alternatively, Li₂ S_(x) isadded to the electrolyte to form the Li₂ S layer on the lithium negativeelectrode in situ. In another embodiment, elemental sulfur is formulatedin the positive electrode mix so that on initial discharge, Li₂ S_(x) isformed and migrates to the lithium electrode where a film is formed. Inany of the above approaches, a tuning agent may be provided with thesulfur or polysulfide additive. Further, the methods may be used incombination to ensure that sufficient additive is available.

Sulfur Additives for Preventing Self-Discharge

Another aspect of this invention provides a system and method forprotecting against self-discharge of a battery. Self-discharge occurswhen the state of charge of a battery's electrodes is reduced by achemical reaction within the cell--while the cell is unconnected to aload.

To better understand self-discharge, consider the reactivity versuscomposition curves in FIG. 3. These curves indicate that as a compound'slevel of oxidation (x+δ) increases, its reaction rate also increases.Nevertheless, the curves change over a wide composition range. So evenat lower oxidation states (corresponding to lower values of x+δ), somereaction occurs. It is this finite reaction rate that causes the cell toself-discharge. Of course, self discharge occurs most rapidly when thecell is highly charged.

To minimize self-discharge, the present invention provides for systemsin which the reaction rate is negligible even at less oxidized states.Such systems are illustrated in FIG. 6, for example. As shown, thereaction rate versus composition curve is very steep, such thatself-discharge is negligible until the electrode composition reachesthat which allows overcharge.

Those of skill in the art will recognize that various of the sulfur andtuning additive compositions described above can be tailored to producepositive electrode material having a steep reaction versus compositionprofile as illustrated in FIG. 6. In one embodiment, this isaccomplished by using a solvent system which leads to such reactivitycurves. To ascertain appropriate systems, corrosion studies may becarried out to determine solvent system and/or tuning agent combinationsin which the reactivity of Li₂ S_(x+)δ with Li₂ S_(x) films on thenegative electrode fall off very sharply with changing values of δ.

Conclusion

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Therefore, the present embodiments are to be consideredas illustrative and not restrictive, and the invention is not to belimited to the details given herein, but may be modified within thescope of the appended claims.

What is claimed is:
 1. A method of protecting against damage fromovercharge, the method comprising:providing a rechargeableelectrochemical energy conversion device including a negative electrode,a positive electrode, and a tuning species which adjusts the rate atwhich oxidized species produced during charging at the positiveelectrode are reduced and thereby adjusts the voltage at whichovercharge protection is provided; and overcharging the rechargeableelectrochemical energy conversion device, whereby one or moreintermediate species of intermediate oxidation state at the positiveelectrode are oxidized to one or more oxidized species of higheroxidation state, which oxidized species move to said negative electrodewhere they are reduced back to said intermediate species, wherein theoxidized species react at a faster rate than the intermediate species atthe negative electrode.
 2. The method of claim 1, wherein therechargeable electrochemical energy conversion device's voltage duringovercharge does not exceed about 2 volts over the device's fully chargedvoltage.
 3. The method of claim 2, wherein the rechargeableelectrochemical energy conversion device's voltage during overchargedoes not exceed about 1 volt over the device's fully charged voltage. 4.The method of claim 1, wherein the rechargeable electrochemical energyconversion device is operated at a temperature of at most above 200° C.5. The method of claim 1, wherein the rechargeable electrochemicalenergy conversion device includes a cell selected from the groupconsisting of lithium/organosulfur cells, lithium/(inorganic sulfur)cells, lithium/(metal oxide) cells, lithium/(metal sulfides) cells, andcarbon anode cells.
 6. The method of claim 1, wherein the tuning speciesis an organic sulfur compound of the general formula (R(S)_(y))_(n),wherein y is a value between 1 and 6, n is a value between about 2 and1000, and R is one or more different aliphatic or aromatic organicmoieties having between 1 and about 20 carbon atoms, which may includeone or more oxygen, sulfur, or nitrogen heteroatoms when R comprises oneor more aromatic rings, or one or more oxygen, sulfur, nitrogen, orfluorine atoms associated with the chain when R includes an aliphaticchain, wherein the aliphatic group may be linear or branched, saturatedor unsaturated, and wherein either the aliphatic chain or the aromaticring may have substituted groups thereon.
 7. The method of claim 1,wherein the tuning species is an alloying element added to a primarymetal component of the negative electrode.
 8. The method of claim 1,wherein the tuning species is a surface active agent selected from thegroup consisting of organoborates, boroxines, polyphosphazenes,phosphates, carbonates, nitrates, and organonitrogen compounds.
 9. Amethod of protecting against damage from overcharge, the methodcomprising:providing a rechargeable electrochemical energy conversiondevice including (a) a negative electrode, (b) a positive electrode, (c)overcharge species produced at the positive electrode which have morethan two oxidation states in which the speed of reaction increases withincreasing oxidation state, and (d) a tuning species which adjusts therate at which the overcharge species produced during charging at thepositive electrode are reduced and thereby adjusts the voltage at whichovercharge protection is provided; and overcharging the rechargeableelectrochemical energy conversion device, whereby the overcharge speciesare oxidized to higher oxidation states at the positive electrode, whichovercharge species of higher oxidization state move to the negativeelectrode where they are reduced back to lower oxidation states tothereby maintain the internal cell potential at a level controlled bythe tuning species in conjunction with the overcharge species.
 10. Themethod of claim 9, wherein the overcharge species is an inorganicpolysulfide or an organodisulfide compound.
 11. The method of claim 9,wherein the overcharge species is a lithium polysulfide or a lithiumorganodisulfide.